All together in one beautiful 3 volume set are C.S. Lewis' "Out Of The Silent Planet", "Perelandra", and "That Hideous Strength.
If you are primarily interested in religious fiction, and have the patience to read books with more complexity than, say, the Left Behind series, you will like these allegorical journeys through the fall of man. If you are primarily interested in SciFi, CS Lewis takes you to other worlds (Silent Planet, Perelandra) and introduces beings from another Earth-time (Hideous Strength) with an original twist of the good vs. evil storyline.
All three books can be read on their own, however I found that "That Hideous Strength" would have been difficult to follow without the background provided in either "Out of the Silent Planet" or "Perelandra". Regardless of the individual readability of the 3 stories, I started with the 1st book (Out of the Silent Planet) not sure I would enjoy it, and ended up finishing all 3 within a week or two.
OUT of the SILENT PLANET introduces Ransom.... Lewis' surrogate voice of traditional Reason tempered by Faith...who is kidnapped by contemporary forces of (literally) superstitous adherrence to Modern/Post-Modern radical secularism and Scientism. Taken to "Mars"((to be victim of blood sacrifice to Alien Intelligences)) Ransom is made aware of the consequences of ORIGINAL SIN that made Earth, THE SILENT PLANET; and battle ground for Cosmic Powers of Good and Evil.
PERELANDRA, in my estimate the most fascinating of the three books, is modern re-Telling of The GARDEN OF EDEN parable. Here Ransom and a character named Weston (believed by some scholars to represent H.G.Wells)become locked in spiritual, psychological and ultimately physical combat to preserve or corrupt an INNOCENT WORLD. C.S.Lewis description of "Venus"...the Edenic Planet...is startling and unique. The battle between the Two WORLD VIEWS is exciting and thought-provoking.
THAT HIDEOUS STRENGTH is scary...
In The Trilogy's conclusion, Lewis(again through his hero Ransom) posits the Forces of Modern/ Post-Modernism are an anti-Faith in league with Demonic Powers. This "Fairy Tale for Adults" imaginatively evokes the myth of the Tower of Babel combined with Arthurian Legends, and New Age Occultism to produce a story of...until the climax when demonic influences are overtly revealed...subtle horror. The N.I.C.E. is described as a scientific United Nations devoted to establishment of a utopian Order of World Peace and Prosperity.